Coated nickel-base and cobalt-base alloys having oxidation and erosion resistance at high temperatures



Nov. 11, 1969 F. P. TALBOOM, JR.. ET AL 3,477,831

COATED NICKEL-BASE AND COBALT-BASE ALLOYS HAVING OXIDATION AND EROSIONRESISTANCE AT HIGH TEMPERATURES Filed Jan. 27, 1966 2 Sheets-Sheet l OOHm0 kmmk 2050mm 022,400 mm 9 3 w zm ommm @ZEHOO M 5286 zmomnrn INVENTORSFRANK P. TALBO0M,JR. JOHN A. PETRUSHA BY Km, 62 Fenders;

ATTORNEYS Nov. 11, 1969 p, TALBOOM, JR, ET AL 3,477,831

COATED NICKEL-BASE AND COBALT-BASE ALLOYS HAVING OXIDATION AND EROSIONRESISTANCE AT fiIGI-I TEMPERATURES Filed Jan. 27. 1966 2 Sheets-Sheet 2A| O SURFACE FILM Tu,Al,Co AND Ni-ALLOY COATING ZONE AI-RICH PHASES-To-AND W- RICH PHASES Ni-ALLOY SUBSTRATE FIG. 2

Al O SURFACE FILM-i AI-RICH PHASES-- To,Al,Co AND Co-ALLOY COATING ZONETo-AND-W-RICH j PHASES Co-ALLOY SUBSTRATE FIG- 3 161m INVENTOR-S FRANKP. TALBOOM,JR JOHN A. PETRUSHA BY Znne qan &- @na erson ATTORNEYS UnitedStates Patent Office 3,477,831 Patented Nov. 11, 1969 3,477,831 COATEDNICKEL-BASE AND COBALT-BASE ALLOYS HAVING OXIDATION AND EROSIONRESISTANCE AT HIGH TEMPERATURES Frank P. Talboom, Jr., Glastonbury, andJohn A. Petrusha, Marlborough, Conn., assiguors to United AircraftCorporation, East Hartford, Conn., a corporation of Delaware Filed Jan.27, 1966, Ser. No. 523,377 Int. Cl. B44c 1/02; C23c 3/04 US. Cl. 29-1959 Claims ABSTRACT OF THE DISCLOSURE This invention relates to novelcoatings for nickel (Ni)- and cobalt (Co)-base alloys that will protectsuch alloys from oxidation at high temperatures, and to a method forcreating such coatings.

More particularly, this invention relates to a tantalum (Ta)- modifiedaluminum (AD-base coating for both Nibase alloys having Ni as theirprincipal component and Co-base alloys having Co as their principalcomponent. The coatings of this invention are created by first forming athin, uniform Ta containing layer on the surface of the substrate by avapor deposition, pack cementation, or other suitable processes. Theseprocesses produce deposition of Ta on the substrate surface, forming athin Ta-rich surface zone metallurgically bonded to the base metal ofthe substrate. An intimate mixture of Al-base powders, preferably amixture of Co and Al powders, is then deposited on the surface of thesubstrate by dipping, painting or spraying it on in the form of a slurryor dispersion in an organic solvent. The powder covered substrate isthen heat treated in a reducing, inert or vacuum atmosphere furnace tocause interdiffusion of the Al and preferably Co powder mixture into thesubstrate surface, resulting in the production of a final coating zoneconsisting essentially of Al, Ta, Co, and the base metal of thesubstrate.

The coatings of this invention provide excellent longterm protection toNiand Co-base substrates at metal temperatures up to 2000 F.,medium-term protection at metal temperatures up to 2100 F., andshort-term protection at metal temperatures up to 2200 F. or more underconditions of high velocity gas erosion such as typically areencountered in a gas turbine engine.

It is noteworthy that the coatings of this invention provide highlysuperior protection for both Ni-base alloys and Co-base alloys.

Although Ni-base alloys typical of those in current use do not begin tomelt until the temperature of about 2380 F. is reached, such alloys ingas turbines, if unprotected, fail rapidly at turbine inlet temperaturesof 1800 F. or above. The mechanism of failure is by preferentialinter-granular oxidation attack at grain boundaries or by general orgross oxidation. Penetration at grain boundaries leads to notches in theloci of penetration, and stresses created at these notches in turn canlead eventually to mechanical failure of the part. An important functionof the coatings of this invention is to prevent such inter-granularoxidation attack on Ni-base alloys.

Co-base alloys generally present more serious oxidation problems thanNi-base alloys. The Co-base alloys are generally subject to highertemperatures in use than their Ni-base alloy counterparts. Co-basealloys thus are sub ject to more rapid general oxidation attack.

Virtually since the introduction of jet aircraft engines during WorldWar II pressures have existed for their constant upgrading. Theimportance for upgrading has been in large part created by the fact thatslight increases in turbine inlet temperatures can provide significantincreases in thrust. In turn, slight increases in thrust yield importantincreases in engine efliciency and economy, but as turbine inlettemperatures are increased performance requirements for engine partsbecome much more demanding. Current engines using parts with availablecoatings are rated at turbine temperatures of about 1900" F. Moreadvanced engines being made today operate at turbine inlet temperaturesof about 2000 F. for constant operation. Work is being done on enginesdesigned to run at turbine inlet temperatures of about 2100 F.

Metal temperatures of turbine blades are nominally 250 to 300 F. belowturbine inlet temperatures in a given engine, but hot spots can becaused in blades and they may go through heat zones that causes them toreach turbine inlet temperatures. Momentary engine overshoots, or suddenbut brief increases in turbine inlet temperatures caused, for example,by large thiust demanded on take-off or by a spurt of fuel admitted tothe combustion chamber at anytime during operation, can result inincreases in turbine inlet temperatures of as much as 300 F. aboveconstant operation temperatures. Such overshoots can cause correspondingtemporary increases in turbine blade metal temperatures of about 300 F.above normal. The clear need thus exists for higher temperature coatingsthat will give good protection to Ni-base and Co-base alloys at metaltemperatures up to at least 2100 F.

Further, the coatings of this invention. are designed for use on bothturbine blades and turbine vanes. The turbine vanes can reachtemperatures as high as 200 F. above turbine inlet temperature. Thus, itwill be seen that oxidation protective coatings at extremely hightemperatures are greatly needed in the industry.

The highest turbine inlet temperatures that uncoated blades can standwithout rapid failure due to high velocity gas erosion is about 1800 F.Better existing coatings, such as Al-10 Si, will protect blades atturbine inlet temperatures up to about 1900 F. By contrast, however, thecoatings of this invention give superior protection to Ni-base alloyblades against high velocity gas erosion for times up to 5,000 hours ormore at turbine inlet temperatures of at least 2100 F. The coatings ofthis invention will also give short time protection to Ni-base alloys atturbine inlet temperatures up to at least 2400 F., thus affordingreliable protection against momentary engine overshoots.

Existing coatings such as the Al-10 Si coating discussed above arecompletely unsatisfactory on. Co-base alloys, and do not provideadequate protection even up to turbine inlet temperatures of about 2000F., at which temperature severe spalling problems may occur. However,the coatings of this invention afford long time protection to suchCo-base alloys at much higher turbine inlet temperatures of up to around2200 F.

Expressed in terms of metal temperatures, the coatings of this inventionprovide protection for Ni-base alloys and Co-base alloys at temperaturesup to about 2200 F. for times of about 50 hours with ability to protectat even higher temperatures for shorter periods of time. They achieve acoating life of over 400 hours at metal temperatures of about 2000 F. onNi-base alloy and a coating life of over 300 hours at metal temperaturesof about 2000 F. on Co-base alloys. The coatings of this invention havea life of many thousands of hours on both Ni-base and Co-base alloys atmetal temperatures of about 1800" F.

Stated more generally, it has been found that the coatings of thisinvention provide oxidation resistance to Nibase alloys and Co-basealloys at temperatures about 100 F. in excess of the temperatures towhich such protection is afforded by any previously known coatings forsuch alloys, and provide protective coatings lives twice as long thethose provided by any such known coatings at any given temperature.

As engine temperatures go up, problems multiply. This invention meetsthe need for a superior coating that will fulfill the requirementsimposed by higher engine operating temperatures. The final product ofthis invention achieves both a high surface melting point andoutstanding oxidation resistance. In protecting turbine blades and vanesat higher operating temperatures, melting temperatures of coatings areof considerable importance. Basically, coatings must be oxidationresistant. However, once oxidation resistance is achieved, therelatively low melting points of some prior aluminide type coatings canbecome a severely limiting factor preventing further increase in turbineinlet temperatures. There has thus been a long-felt need for coatingshaving both superior oxidation resistance and high melting points thatwould be able to withstand the exigencies of higher engine operatingtemperatures without failure.

Existing coatings furnish adequate oxidation resistance at turbine inlettemperatures up to 1900 F., but when the turbine inlet temperature ismoved up to 2100 F, these coatings become subject to melting by exposureto hot spots or momentary engine overshoots. Characteristically,coatings on Ni-base and Co-base substrates tend to soften attemperatures below their melting points. The closer the melting point isapproached, the softer the coating becomes. As exposure temperatures ofcoatings are increased, erosion is accelerated by softening of thecoating. Coatings thus can be caused to fail by gross erosion whenexposed to high velocity turbine gas at temperatures appreciably belowtheir metling points, a characteristic that again emphasizes theimportance of a high melting point for a satisfactory coating.

A concomitant problem has been to achieve a coating that in spite of itshaving a high melting point can nevertheless be applied at a temperaturethat is compatible with the heat treating temperature of the Ni-base andCo-base substrates. In most Ni-base alloys and Cobase alloys for turbineblades and vanes a good temperature for initiation of heat treating isabout 1975 F. Ideally, then a coating for such blades should be capableof being applied at this temperature. It is a beneficial result of thisinvention that the coatings taught can be applied at the relatively lowheat treating temperatures characteristic of Ni-base and Co-basesubstrates but still yield coatings having much higher melting pointsthan their application temperatures.

The improved oxidation protective coatings of this invention havedesirably high melting points in excess of the maximum temperaturelimits to which existing Nibase and Co-base alloys can be exposedwithout en countering melting or unacceptable softening of the substrateitself.

Generally, as engine operating temperatures are increased, oxidationresistance is lower; erosion is increased; and more frequent inspectionand replacement of engine parts is required. Although they extend thelife of engine hardware, current production coatings do not providetheprotection and longevity required for extended use of engines in the1800-2000 F. turbine inlet temperature range. Such coatings areinadequate for these high engine operating temperatures because as theturbine inlet temperature is raised these coatings display the followinginadequacies (1) Excessive interdiffusion between coating and substratetakes place with consequent dilution of coating composition and loweringof its protection potential.

- (2) Melting points of existing coatings are close to metaltemperatures experienced in higher temperature engines.

(3) At such temperatures existing coatings offer insufiicient oxidationresistance.

(4) When their melting points are closely approached, such coatingssuffer from excessive gas erosion by the turbine gas stream.

(5) Some existing coatings, when applied to Co-base alloys, becomehighly susceptible to spalling as engine temperatures are increased.

(6) As higher engine temperatures are used more rapid interdiffusionbetween the coating and the substrate occurs, diffusing components suchas Al from the coating into the substrate, leaving no A1 at the surfacefor the formation of Al-oxide or other protective oxide films on theouter surface of the substrate, which oxide films provide the primaryoxidation resistance of the coatings.

(7) As exposure to high temperature oxidation continues, the adherencebetween the Al-oxide (A1 0 pellicular film on the outer surface of thecoated substrate and the remaining portion of the coating deteriorates.Since the primary oxidation resistance of the coating is afforded bythis outer A1 0 film, the loss of adherence between this film and thecoated substrate greatly reduces the effectiveness of the protectionafforded by the coating, and renders the coated article subject toextensivb oxidation attack.

In view of the foregoing, it is a primary object of this invention toprovide as a new and improved article of manufacture a Ni-base orCo-base alloy substrate having an oxidation protective coatingmetallurgically bonded thereto comprising a Ta-rich coating zone locatedbetween the substrate and an aluminum oxide (A1 0 outer coating filmlocated at the surface of the coated composite, which article achievesgreatly improved adherence between the outer A1 0 oxidation protectivecoating surface film and the remainder of the coated Ni-base or Co-basecomposite article, and to provide a process for producing such anarticle.

Another object of this invention is to provide for Ni-base and Co-baseallows a new and improved Tamodified Co-Al coating composition that hasa melting point in excess of the upper limit of temperatures to whichexisting Ni-base and Co-base alloys can be exposed without melting orunacceptable softening of the substrate.

It is another object of this invention to provide a superior coating forNi-base and Co -base alloys that can be applied by heat treatment atrelatively low temperatures, but when once applied will have a meltingpoint well above such temperatures.

A further object of this invention is to provide a new and improvedcoating composition for Ni-base and Co-base alloys that has a highmelting point and also possesses room temperature ductility. The lattercharacteristic of such coatings makes them capable of deforming withindentations or defects imposed on the coated parts, thus making thecoatings resistant to failure from ballistic impact at low temperatures.

Another object of this invention is to provide a new and improvedcoating for Ni-base and Co-base turbine blades and vanes that willenable them to be operated at temprtatures where they can perform moreefiiciently and still be protected from failure through inter-granularoxidation attack.

Yet another object of this invention is to provide a process forapplying a Ta-modified Co-Al coating composition to Ni-base and CO-basealloys, which process achieves oxidation protective coatings on suchalloys having more uniform Ta content than has been heretofore possible,and results in greatly improved adherence of the A1 0 coating surfacefilm to the remainder of the coated composite.

A still further object of this invention is to provide an improvedprocess for applying Ta-modified Co-Al coatings on Ni-base and Co-basealloys which results in the production of improved oxidation protectivecoatings on such alloys having vastly superior adherence between the A10 outer coating surface film and the remainder of the coated composite;which adherence provides superior high temperature oxidation resistanceto the alloys coated by this process.

Additional objects and advantages will be set forth in part in thedescription that follows, and in part will be obvious from thedescription, or may be learned by practice of the invention, the objectsand advantages being realized and attained by means of the compositions,methods and processes, particularly pointed out in the appended claims.

To achieve the foregoing objects and in accordance with its purposes,this invention in a preferred embodi ment provides an article ofmanufacture having goodstress rupture strength at high temperatures,high-temperature oxidation resistance, and resistance to cyclic thermalfatigue failure which comprises a substrate conisting of essentially aNi-base alloy or a Co-base alloy, the article having a defect,oxidation, interdifiusion, thermal shock, melting, and erosion resistantsurface zone metallurgically bonded to the surface zone and consistingessentially of a Co-Al composition having atomic ratio of Co to Al from2:5 to 1:1, which Co-Al composition is modified by from 0.1 to 1.0% byweight of the surface zone of Ta, the surface zone being furthercharacterized by ductility at room temperature, and a melting pointhigher than that of the substrate. I

It should be understood that the coating zone of this invention consistsessentially of an interdiffusion product of the base material of thesubstrate, Ta, which is applied in the first process coating step, andCo and Al which are applied in the second process coating step.Therefore, the composition of the coating zone will consist essentiallyof Co, Al, and Ta, as described in the above embodiment, only when thesubstrate being coated is substantially pure Co. If the substrate issubstantially Ni, then the coating zone consists essentially of aCo-Ni-Al composition having an atomic ratio of Co-Ni to Al or from 2:5to 1:1, which Co-Ni-Al composition is modified by from 0.1 to 1.0% byweight of the surface zone of Ta.

In like manner, where the substrate is a C0 alloy or a Ni alloy, thesurface zone will consist essentially of the particular alloy of thesubstrate together with the Co of the second coating composition in anatomic ratio to A1 of 2:5 to 1:1, with the overall Co-Ni alloyAl orC0-C0 alloy-Al composition being modified by from 0.1 to 1.0% by weightof the surface zone of Ta.

In a more general form this invention provides an oxidation-resistantarticle having a Ni-base or Co-base substrate and an oxidation anderosion resistant coating zone metallurgically bonded to the substratewhich comprises a Ta-rich subzone adjacent to the substrate, and analuminum-base outer coating zone having an oxidation resistant A1 0surface coating film adherently bonded to the remainder of the coatedcomposite article. The overall coating contains Al, the Nior Co-alloy ofthe substrate and Ta. The Ta is present in the overall coating inamounts from 0.1 to 1.0% by weight. The Ta is deposited on the substratein accordance with the Ta-deposition first step of the process of thisinvention. The A1 0 surface film, located at the outer surface of thecoated article, provides primary oxidation and erosion resistance.However, for this resistance to be effective at the temperatures and forthe time periods contemplated by this invention it is necessary thatthis A1 0 surface film have good adherence to the remainder of thecoated composite. Such adherence has been found to be achieved byinterposing a Ta-rich coating subzone between the substrate and theAl-base surface coating zone which provides the Al for the formation ofthe A1 0 surface film. Thus the Ta-rich subzone promotes the superioradherence of the A1 0 surface film which is the key to the improvedoxidation and erosion resistance provided by the coatings of thisinvention.

Any Al-base coating composition can be used to supply the Al forformation of the A1 0 surface film, including pure Al and variousAl-based compositions. Al-Co compositions have produced particularlybeneficial results and are the preferred second coating compositions ofthis invention.

As used in this specification and in the appended claims, the termsNi-base alloy and Co-base alloy will be understood to include both pureNi and Co substrates and those alloys in which Ni and Co, respectively,is the principal component and is present in an amount of not less than40% by weight of the alloy.

The invention further comprehends a two-step process for producing acoated metal article having good stressmpture strength at hightemperatures, high-temperature oxidation resistance, and resistance tocyclic thermal fatigue failure, the article comprising a metal substrateconsisting essentially of a Ni-base alloy or a Co-base alloy, and themethod comprising the first step of forming a thin Ta-containing layerat the surface of the article being coated by vapor deposition, paclkcemetation, or equivalent processes, this Ta-containing zone consistingessentially of uniformly distributed Ta and the base metal of thesubstrate; and the second step of contacting the Ta coated substratewith a mechanical mixture of finely divided powders consistingessentially of 5 to 40% by weight of Co, and the balance Al, placing thesubstrate while in contact with the metal powders in an inert, reducingor vacuum atmosphere, and heating the substrate while in contact withthe metal powers to a heat treatment dilfusion temperature of from 1600to 2100 F. for a time period sufficient to create a coating zone on thesubstrate, adherently and metallurgically bonded thereto, which coatingzone has a Ta-modified Co-A1substrate base metal composition.

The first step of the process of this invention is the Ta-depositionstep, resulting in the formation of the Ta-rich coating zone on thesubstrate surface. This step is carried out by a process which willpromote and effect diffusion between the Ta and the substrate, resultingin metallurgical bonding of the Ta-coating zone to the substrate, by theformation of an intermetallic composition between the Ta and the alloyof the substrate. Thus the coating zone produced in this first stage ofthe coating process of this invention consists essentially of Ta and thebase metal of the substrate, i.e., the Coor Ni-alloy. the first stage ofthis process is therefore carried out by vapor deposition using tantalumhalides, by a pack cementation process or by other suitable procedures.

In the vapor deposition process the Nior Co-alloy substrate is heated toa temperature sufficient for the desired interdiffusion of the Ta intothe substrate to occur, generally about 1400 to 2200 F., and at suchtemperature, is exposed to the Ta halide vapors in the presence of areducing atmosphere. This results in the formation of a hydrogen halidegas which is vented from the reaction zone, and the deposition of Ta onthe substrate and its interdiffusion into the substrate to form aTa-Ni-(or 00-) alloy coating zone of the desired thickness,metallurgically bonded to the substrate.

The so-called pack cementation process is a form of vapor deposition, inwhich the object to be coated, i.e., the Nior Co-alloy is surrounded bya particulate pack mixture containing, for example, the metal to bereacted with or deposited on the object to be coated (e.g., Ta), anactivator or energizer (usually a halide salt, such as, NaCl, KF, NH I,NH Cl, and the like), and an inert filler material (e.g., A1 0 SiO BeO,MgO, and the like).

This mixture, held in a suitable container (steel box, graphite boat, orrefractory oxide crucible, for example), is then heated to a desiredcoating temperature, in a prescribed atmosphere, and held for a lengthof time sufficient to achieve the desired coating. In the instantprocess the pack-cementation process is carried out at a temperature offrom 1600 to 2200 F., and preferably at 2000 F. for a time period of twoto 16 hours, and preferably about 4 hours, under a high vacuum,preferably on the order of about 1 micron or less. When conductedproperly, the pack-cementation process will result in acontrolledthickness Ta-containing coating on the Coor Ni-base alloysubstrates coated in accordance with this invention. The coating zoneson these substrates consist essentially of Ta and the metal or metalalloy of the substrate, and will be metallurgically bonded to thesubstrate by the Ta-alloy intermetallic reaction products formed duringthe Ta-deposition step. The coating zone is characterized by a uniformdistribution of Ta.

The Ta-containing first coating zone produced in the first step of thisinvention generally has a thickness of from about 0.2 mil to about 1.5mil. The thickness of this coating zone is preferably between about 0.4and about 0.5 mil, and a thickness of about 0.5 mil is consideredoptimum.

The use of vapor deposition, pack-cementation or other equivalentprocesses which promote diffusion between the Ta and the substrate inthe first stage of the instant process is important to the effectiveproduction of the coatings of this invention. It may also be possible touse such processes as electroplating or plasma-spraying to apply the Tain the first stage of this process, provided the Ta can be subsequentlyproperly diifused into the surface of the substrate by heat treatment.However, such processes have not produced coatings equivalent to thevapor deposition and pack cementation steps described above, and hencethe latter procedures are preferred.

Following the formation of the Ta-substrate alloy coating zone, the Tacoated Coor Ni-alloy substrates are preferably subjected to a furtherdiffusion step. Optimum results are generally obtained by subjecting theTa-coated substrateresulting from the vapor deposition orpack-cementation step described above to further diffusion heattreatment at a temperature of about 1600 to 2200 F. for an additional 2to 16 hour period in a vacuum atmosphere of preferably less than 1micron. This optional step results in more complete diflusion of surfaceTa into the coating zone at the surface of the substrate, resulting in amore Ta-rich surface zone. This additional step may also be beneficialin removing any hydrogen dissolved in the Ta.

This optional additional heat treatment diffusion step, like the initialpack cementation or vapor deposition step, need not be carried out undera vacuum, but may be carried out under an argon or other inertatmosphere or under hydrogen. I neither of the latter instances the heattreatment diffusion step will be effected at atmospheric pressure.

After the Ta coating step of the process has been completed, thesurfaces of the tantalum coated article can be cleaned by vapor blast toprepare the article for application of the Co-Al second coatingcomposition. Exemplary of such cleaning is a vapor honing for one minutewith -325 mesh A1 at 40 p.s.i. Use of this vapor blast rather than amore conventional dry grit blasting with a heavier A1 0 grit minimizesthe chance of stripping the Ta-coating zone from the substrate surfaceduring cleaning. It is preferable to omit this surface cleaning step ifpossible, i.e., if a surface receptive to the subsequent Co and Alcoating step can be presented without the need of surface cleaning.

Of course, subject to limitation of such possible stripping, and thepreferred omission of any cleaning step, the Ta coated substrate surfacecan be cleaned by any conventional technique for removal of dust or dirtparticles, such as by water rinsing, liquid blasting, washing insuitable organic and inorganic solvents, and any other method ofcleaning that is standard in the art. As pointed out above, care shouldbe taken in cleaning the Ta-coated substrate to insure that it is notinjured. It will be appreciated that any of the above standard cleaningprocesses can also be used to clean the Coand Ni-alloy substrates priorto the initial Ta coating stage of the process.

The C0 and Al powders preferably used as the second coating composition,in the second step of the process of producing the coatings of thisinvention, usually have a size range of less than 325 mesh (43 microns)although coarser particles, ranging in size from about mesh (147microns) to 325 mesh may also be used. Especially good results areobtained when the size range of the Co and Al powders is less than 400mesh (38 microns), or between about 0 to 38 microns, and preferablybetween about 0 to 10 microns. In general, it can be said that the finerthe particles, the better the coatings produced. The mesh sizes referredto above are Tyler Standard.

The metallic dust or powders of Co and Al described above can be appliedto the Ta coated Ni-base or Co-base alloy part, metal core, orsubstrate, to be treated in any suitable manner. A fine film of the Coand Al powders can thus be blasted or dusted onto the specimen; or adispersion of the powders in a solvent liquid can be applied to thesubstrate, after which the solvent can be evaporated leaving a coatingof the powder mixture on the substrate. Other methods of applying the C0and Al powder mixture will readily suggest themselves to persons skilledin the art.

In accordance with the preferred embodiment of this invention, a C0 andAl powder mixture is dispersed in a suitable liquid dispersant, and theresulting dispersion is applied to the substrate by spraying, brushing,dip-coating, or any other conventional method.

The ratio of Co and Al powder mixture to liquid dispersant may vary fromabout to 5% by weight or higher. The liquid dispersant can be verysuitable, readily volatilizable organic solvent, or mixture of solvents.Among the solvents that can be used are alcohols, such as, methyl,ethyl, propyl, and butyl alcohol, esters such as methyl, ethyl, propyl,butyl, and amyl acetate, and ketones, such as, for example, acetone.

The organic solvents mentioned are illustrative and not limiting. Itshould be understood that almost any volatile liquid that will act as asuitable dispersant for the Co and Al powder mixture can be utilized,and any such liquid is contemplated. The main requirement of thevolatile liquid substance or dispersant is that it be reasonably safe touse, inexpensive, and sufficiently liquid at ordinary temperatures toact as a dispersant for the metallic powders so that the dispersion canbe sprayed or suitably coated on the specimen, and at the same time besufficiently volatile to evaporate when exposed to atmospheric or otherconditions as will be described below.

If desired, a binder or sticking agent can be added to the liquiddispersant to hold the powder mixture to the surface of the substrateafter evaporation of the solvent. Use of a binder enables the powders toadhere to the substrates for prolonged periods of time, therebyprecluding the necessity of heat treating immediately after applicationor of taking special precautions in handling the treated substrate. Thebinder should be one that will be substantially completely decomposedduring diffusion heat treatment or at a temperature below actualdiffusion heat treatment temperature. Suitable binding and stickingagents that can be used include nitrocellulose, naphthalene, andstearates. Other sticking or binding agents will be readily apparent tothose skilled in the art.

Suitable wetting agents can also be added to the dispersant if required.

The dispersion of Co and Al powder described above in either a liquid orlacquer dispersant, i.e., a dispersant co taining a binder or stickingagent, is deposited on the surface of the specimen to be coated in themanner already described. After application, the solvent is allowed toevaporate, thereby leaving a layer of Co and Al powder mixture on thesubstrate. As pointed out above, the second coating composition, inaccordance with the broadest teachings of this invention, can be pure Alor any Al-base composition suitable for formation of the A1 surfaceoxidation protective film. However, the invention is here described interms of the use of the preferred Co-Al second coating composition.

If a sticking agent is added to the dispersant, upon evaporation of thesolvent, the sticking agent will remain dispersed throughout the dust orpowder in the coating and will serve to hold the powder or dust to thesubstrate.

Evaporation of the volatile solvent or volatile portion of the lacquercontaining a sticking agent can be conveniently brought about byallowing the coated substrate to be stored in an atmospheric environmentat ordinary temperatures. If desired, suction or vaccum and elevatedtemperatures can also be used to accelerate evaporation of the volatilesolvent. Evaporation of the solvent leaves a fine layer of Co and Alpowder mixture on the surface of the substrate including any walls orsides defining interstices, slots, holes, and so forth, that may bepresent in the substrate.

No separate step is necessary for evaporation of the solvent from thepowder dispersion after its application to the previously Ta coatedsubstrate. After the dispersion is applied to the substrate, it can beimmediately heat treated, and the solvent will be flashed off orevaporated during this heat treatment.

When a hinder or sticking agent is added to the liquid dispersant, thecoating layer, upon evaporation of the solvent, comprises a uniformintermixture powder interspersed throughout the nonvolatile hinder orsticking agent. The dried coating adhering to the specimen comprisesmetallic particles and hinder, the metallic powder being suspended in orinterspersed throughout the binder.

Preferably the mixture of cobalt and aluminum powders is formed into aslurry with the dispersant or binder or sticking agent. The substratecan then be dipped into the slurry or the slurry can be sprayed orbrushed on to the substrate. The substrate in turn can be masked inselective areas to prevent adherence of the slurry or dispersion to suchmasked areas and to prevent the formation of any coating on such areasduring subsequent heat treatment. These same areas can also be maskedduring the previous Ta-deposition step or the Ta applied in that stepcan be removed from the surfaces to be masked during the Co and Alcoating application by grinding, or other machining techniques.

The amount of Co and Al powders applied to the Tacoated substrate canvary from substrate to substrate. But in general, an amount betweenabout milligrams per square centimeter of substrate area and about 30milligrams per square centimeter of substrate area is contemplated. Suchamounts result in production of a coating of the desired thickness afterheat treatment of the Co and Al second coating composition.

Preferably after allowing the solvent to be completely evaporated fromthe substrate, the resulting specimens are heat treated in a suitablefurnace or oven to cause diffusion of the Co and Al into the previouslyTa modified substrate surface zone, thereby producing the improvedcoatings of this invention. Heat treatment temperatures of from 1600 to2100 F. are used in this heat treatment step, and temperatures of 1950to 2000 F. are preferred.

The heat treatment period can vary from about 1 hour to 20 hours ormore. Particularly good results are achieved when the heat treatment iscarried out for about 4 hours.

In accordance with the present invention, superior coatings are producedby heat treatment of the Co and Al second coating composition in ahydrogen atmosphere furnace. The diffusion is carried out underatmospheric pressure, or preferably at a pressure slightly greater thanatmospheric. The hydrogen atmosphere is particularly critical and shouldhave a maximum dew point of 40 F. or less, preferably 60 F. 'It isimportant that the hydrogen be as completely free of oxygen as possible.

Although a high purity hydrogen atmosphere furnace is preferred for thefinal heat treatment step used in producing the coatings of thisinvention, it is also possible to use a high purity argon or other inertatmosphere or a vacuum atmosphere heat treatment step.

The second coating composition of this invention in its preferred formcontains 5 to 40% by weight of Co and 60 to by weight of A1. A11 optimumcomposition contains 80% by weight of Al and 20% by weight of Co.

The final coating produced by the two step process of this invention,described above, consists essentially of Co, Al, Ta, and the base metalor metal alloy of the substrate. This coating zone has a thickness offrom 2 to 6.5 mils. A coating thickness of about 5.5 mils is optimum.

Of the essence of the present invention is a dramatic and unexpectedincrease in the effectiveness of Al-base coatings on Niand Co-base alloysubstrates achieved by first forming on the substrate a thin, uniformTacontaining coating Zone, and subsequently applying and interdiffusinginto the Ta-containing coating zone an Albase coating composition,preferably consisting essentially of Co and Al.

The resulting coating consists essentially of Co, Al, Ta, and the basemetal or alloy of the substrate. The coating thus preferably contains Cofrom the second coating composition, and a Co-alloy or Ni-alloy from thesubstrate, in aggregate, in an atomic ratio to A1 of from 2:5 to 1:1,and the Co-alloy-Al composition is modified by 0.1% to 1.0% by weight ofthe coating zone of Ta.

The coatings produced in accordance with this invention have a moreuniform distribution of Ta throughout the coating zone, and hence ahigher useful Ta content than can be achieved by any previously knownprocess. Comparable coatings can not be obtained merely by applying amixture of Ta, Co, and Al powders on the sub strate and subsequentlyheat treating. Such coatings were produced on Mar-M200 Ni-base alloysusing a coating composition having a Co:Ta:Al ratio of 1:318, by weight.The as-coated articles were found to contain no Ta at or near thecoating-substrate interface.

The Ta modifier uniformly present in the coatings of this inventionproduces unexpected beneficial results, including increasing thediffusional stability of the coating, and most importantly inunexpectedly improving the adhesion between the A1 0 surface oxide layerformed on the coating during oxidative exposure and the remainder of thecoated composite, thereby greatly improving the oxidation resistance ofarticles produced in accordance with this invention. These coatingsprovide oxidation protection at temperatures F. in excess of anytemperatures to which equivalent protection is provided by existingcoatings for Niand Co-base alloys, and provide endurance life andoxidation exposure of twice as long as that provided by previouslyavailable coatings for such alloys.

In addition, the coatings of this invention are not susceptible to theacute spalling problems which have heretofore been encountered inattempts to provide oxidation protective coatings for Co-base alloys.

It is important that the cobalt and aluminum powders used in forming thecoatings of this invention be of the highest purity obtainable. Co andAl powders should be of 99% or greater purity. Inclusion of even smallamounts of silicon (Si) may prove undesirable. Even though grossoxidation resistance may not be affected, Si may cause unacceptablereduction in the melting point of the coating through introduction oflow melting phases between Al and Si as well as between Ni or C and Si.Si may also adversely affect the ductility of the coatings produced.

Titanium (Ti) is also preferably avoided, since it confers no benefit tothe coating and may lower its heat resistance. Ti may also tend todegrade the beneficial diffusion arresting effects that Ta has on Al. Ifmetal powders of the highest purity obtainable consistent with economicfactors are used, the danger of undesirable side effects from additionalelements introduced as impurities is greatly reduced.

For a clearer understanding of the invention, specific examples of itare set forth below.

EXAMPLE 1 A Ni-base alloy called Mar-M200 and having the followingnominal composition by weight:

Ni-12.5W-10Co-9Cr-5Al-2Ti-1Cb-0.15C-0.05Zr-0.0l5B is subjected to a packcementation process as follows. An alloy specimen is surrounded by aparticular pack mixture containing Ta, NH Cl and A1 0 which mixture isheld in a graphite boat. The pack, containing the alloy specimen is thenheated to a temperature of 2000 F. at a vacuum of about 1 micron andheld at that temperature for about 4 hours.

During this treatment the Ta reacts with the Ni-alloy substrate to forma surface zone about 0.5 mil thick containing intermetallic reactionproducts of Ta and the alloy of the substrate.

The alloy specimen was prepared for the pack cementation process by gritblasting its surface with No. 60 A1 0 grit at 40 p.s.i. for 2 to 5minutes, followed by degreasing with trichlorethylene at 180 F. for 5 tominutes.

After the pack cementation process resulting in the formation of the 0.5mil thick Ta containing surface zone on the alloy substrate wascompleted, the Ta-coated alloy specimen was subjected to a further heatdiffusion step by heating it at a temperature of 2000 F. for 4 hours ina vacuum furnace at a pressure of less than 1 micron.

The surface of the Ta-coated alloy specimen was then cleaned by vaporhoning for 1 minute with --325 mesh A1 0 at p.s.i. The specimen was thenready for application of a Co-Al second coating composition.

A mixture of high purity metallic powders of the following compositionwas prepared:

grams of analytical grade Co powder (325 mesh or finer) 200 grams flakeAl powder (7 microns) The following liquid dispersant was also prepared:

680 milliliters nitrocellulose lacquer, Pratt and Lambert No. 2012(primarily amyl acetate and nitrocellulose binder).

A ball mill container was filled with a minimum of 5 pounds of 1 inchdiameter porcelain milling balls or enough balls to fill the container/3 full. A measured quantity of Co and Al powder mixture was then placedin a container and a measured amount of liquid dispersant was addeduntil the balls, powder and liquid in the container filled it from /2 to/s full. The contents of the ball mill were then milled to a slurry forfrom 8 to 16 hours at about 14 r.p.m.

For good results the viscosity of the slurry was kept at between about600 and 1000 cps. at to 85 F. as measured with a Brookfield Viscometerusing the No. 1 spindle at 10 r.p.m. or equivalent. If necessary, theviscosity was reduced by adding additional dispersant and mixingthoroughly once more, either by ball milling or rotating the containerwithout milling media for approximately 1 hour. If the viscosity was toolow it was increased by adding additional Co and Al powder mixture andmilling as described or by blending with a slurry of a higher viscosityand milling, as described, for 1 hour.

The resulting dispersion was then sprayed onto the previously Ta-coatedNi-alloy specimen.

The solvent was evaporated by allowing the pecimen to stand at roomtemperature. Following evaporation of the solvent, the specimen, with itadhered applied powders, was placed in a hydrogen atmosphere cyclicfurnace at 2000 F. High purity hydrogen having a dew point of about 60F. was introduced into the furnace to a pressure slightly exceedingatmospheric.

Diffusion heat treatment was carried out on the coated Ni-alloy specimenfor 4 hours at 2000" F. and under a pressure of hydrogen slightlygreater than atmospheric.

After the diffusion heat treatment was completed, the hydrogenatmosphere was maintained and the coated specimen was cooled to 500 F.It was then removed from the furnace and allowed to cool to roomtemperature.

The resultant alloy article had an interdiffused coating zone about 5.5mils thick adherently bonded to the alloy substrate consistingessentially of Co, Al, Ta and the Nialloy of the substrate.

The coating of this example was subjected to a dynamic oxidation testingenvironment, i.e., to flowing air at 2100 F., for hours. During thistime no coating failure was observed. To graphically illustrate thesuperiority of the coatings of the present invention over well knownexisting coatings, erosion bar specimens made up according to Example Iwere simultaneously oxidation-erosion tested with certain commerciallyavailable coatings on similar alloy substrates. The coatingcompositions, substrate compositions and heat treatment time" gt] thesevarious test samples are set forth in Table I e ow:

TABLE I Specimen Coating Substrate Heat No. composition compositiontreatment 1 Example I. Ni-12.5 W-lO Co-E) Cr- 2,000 F., 4 hours 5 Al-2Ti-l Cb- 0.15 (hydrogen). C 0.05 Zr-0.015 13. 2 Example II. (Jo-20 Cr-15W-lO Ni- 1,800" F., 4 hours llllg-Ol C- 3 Fe- (hydrogen).

1. 3 Chromalloy Co-21.5 (Jr-10 W-9 Ta- Pack-0e entati n.

UC. 1.5 Ni-l Fe-0.86 C- m o 0.25 Zr. 4 PWA-47 Ni-18.5 00-15 Cr-B 1,975E, 4 hours M o-4.3 Al- 4 l fe-3.3 (hydrogen). 313E 01 Cu-0.07 (3-0433 5Example I do. 2,000 F., 4 hours (hydrogen).

1 Mar-M200, cast Ni-base alloy, Martin Metals Co.

2 L-605, wrought (Jo-base alloy.

3 Al-base coating, deposited by pack cementation.

4 Mar-M302, cast Co-base alloy, Martin Metals Co.

{Al-10S) (by weight) coating, deposited by slurry spraying withdiffusion heat treatment.

Udimet 700, wrought N i-base alloy.

Both of the conventionally coated alloy specimens failed due togeneralized oxidation prior to reaching 100 hours of oxidation testing.This is clearly illustrated by visual examination illustrated by FIGURE1 which shows general erosion of all the coated specimens of Table Iexcept for the three bars coated in accordance with the teachings ofthis invention and designated Example I and Example II in FIGURE 1. TheExample I and Example II bars were still in excellent condition after100 hours of dynamic oxidation testing.

FIGURE 2 is a photomicrograph of a trailing surface of the Ta and Co-Alcoated Mar-M200 Ni-base alloy erosion bar produced in accordance withExample I, and enlarged 500 times to show the composition of the coatingafter oxidation-erosion testing in flowing air for 100 hours at 2100 F.This photomicrograph (FIG. 2) shows the Al O surface film at theexterior surface of the coated article and underneath this a coatingzone consisting essentially of Ta, A1, C0 and the Ni-alloy of thesubstrate. At the substrate-coating interface the metallurgical bondingbetween the substrate and the coating zone is clearly shown. Ta-rich,Al-

13 rich, and W-rich phases in the coating are also shown in FIG. 2.

It was noted in carrying out the above comparative oxidation-erosiontests that the thickness of the coatings of this invention was greaterthan the deposited thickness of any of the commercial coatings.Therefore, in order to determine whether this increased coatingthickness was responsible for the improved oxidation resistance of thesecoatings a Ni-base alloy specimen having the composition Ni 125W10Co-9Cr-5Al-2Ti-lCb-0.15C- 0.05Zr-0.015B and a Co-base alloy specimenhaving the composition C 20Cr lW-l0Ni-L5Mn-0.lC- 3Fe- 1Si were eachcoated with a single layer of a CoAl coating applied in the same mannerused to apply the second coating composition of Example I. The coatingin each instance was applied to a thickness of 5.5 mils, and the sampleswere then oxidation erosion tested at 2100 'F. Both of these specimensfailed in less than 100 hours,

the Ni-alloy specimen failing by localized oxidation and the Co-alloyspecimen failing by coating spalling. These tests showed that thegreater coating thickness of the coatings of this invention was notprimarily responsible for the superior oxidation and erosion protectionwhich they afford.

A Ni-base alloy specimen prepared in accordance with the procedure ofExample I was thermal fatigue tested by exposure to a combustion flameof JP-S fuel (a high flash point kerosene-type jet fuel) and airproviding an atmosphere closely approximating that encountered in a gasturbine engine. The sample was initially exposed for 30 minutes at 2000F. to the combustion flame, and was subsequently exposed for 100 cyclesof 1 minute at the 2000" F. temperature followed by 30 seconds at a cold200 F. temperature. The cooling was provided by a cold air blast in theabsence of the flame. Upon completion of the 100 cycles the part wasinspected and the entire process repeated until coating failure or until1400 cycles were completed. The total 1400 cycles consisted 600 cyclesat 2000 F., followed by 400 cycles at 2100 F., followed by 400 cycles at2200 F.

At the completion of the test the samples were examined by opticalmicroscopy.

The Ni-base alloy specimen coated in accordance with Example I completed1500 cycles of testing in this manner (700 cycles at 2000 F.). The basemetal alloy cracked at 1000 cycles, but no coating spalling had occurredat the completion of 1500 cycles.

EXAMPLE 2 In this example a Co-base alloy specimen was coated bysubstantially the same procedure followed in Example 1. This Co-alloyspecimen had the following nominal composition:

This Co-alloy specimen was first Ta-coated by the pack cementationprocess of Example 1. -It was then subjected to further heat treatmentdiffusion of the Ta for 4 hours at 2000 F. in the manner described inExample 1.

A Co and Al powder dispersion in nitrocellulose lacquer, prepared in themanner described in Example 1 was then applied to the surface of the Tacoated Co-alloy specimen in an amount of 25 milligrams per squarecentimeter. This application, as in Example 1, was by spraying.

The coated Co-alloy specimen was then inserted in a hydrogen atmospherefurnace and heat treated by the procedure described in Example 1, exceptthat the heat treatment temperature used in this example was 1800 F. andthe heat treatment was carried out for 4 hours.

The coated alloy of this example was also subjected to dynamic oxidationtesting in flowing air at 2100 F. for

hours, and survived this test with no failure. The superiority of thecoatings of the present invention on Cobase alloys is graphicallyillustrated by FIGURE 1 which shows the specimen produced in Example 2after this 100 hours of oxidation-erosion testing. This sample was stillin excellent condition after the testing.

FIG. 3 is a photomicrograph of a trailing surface of the Ta and Co-Alcoated L-605 Co-base alloy erosion bar produced in accordance withExample 2, enlarged 500 times to show the composition of the coatingafter oxidation erosion testing in flowing air for 100 hours at 2100 F.This photomicrograph (FIG. 3) shows the A1 0 surface film at theexterior surface of the coated article and underneath this a coatingzone consisting essentially of Ta, A1, C0 and the Co-alloy of thesubstrate. FIG. 3 shows the metallurgical bonding at thesubstrate-coating interface and Taand Al-rich phases in the coating.

A Co-alloy specimen produced in accordance with this example wassubjected to thermal fatigue testing by exposure to a combustion flameof IP-S fuel and air in the manner described in Example 1. The specimencoated in accordance with this invention completed 1400 cycles oftesting, and although the base cracked at 500 cycles, no coatingspalling had occurred at the end of the 1400 cycle test.

The exact mechanism of modification or change wrought by the applicationof a thin Ta first coating, in the first coating step of the process ofthis invention, on the final coatings produced in accordance with thisinvention is not fully understood.

After exposure of the specimens of Example 1 and Example 2 to dynamicoxidation environments at 2100 F. for 100 hours, the coating zones ofthese specimens were analyzed and found to contain less than about 0.5%Ta. It is believed that the Ta which remains in the surface zone aftersuch oxidative exposure has a more uniform distribution than that whichis achieved by applying Ta by procedures other than the present process.This uniform Ta content greatly promotes the adherence of the outeroxide pellicular film of A1 0 to the remainder of the coated composite,and it is this outer A1 0 film which provides the primary oxidationresistance of the coating.

Analysis of the coatings of this invention after oxidation testing alsorevealed that an extremely high concentration of tungsten from the alloysubstrate had formed at the coating substrate interface of the NiandCo-base alloys coated in accordance with this invention (as shown inFIGS. 2 and 3), and that a change in chemistry had occurred in alight-etching micro constituent at the outer edge of the coatingsproduced in accordance with this invention on the Ni-base alloyspecimens. Some or all of these unexpected occurrences may contribute tothe improved oxidation resistance aiforded by the coatings of thisinvention. Without being bound to any particular theory, applicantsbelieve that the primary basis for the improved coating performanceresults from improved adherence of the outer A1 0 coating film to theremainder of the coated composite, and that this improved adherenceresults from the process of this invention in first depositing a uniformTa coating layer on the Nior Co-base alloy substrate prior toapplication and interdiflusion of the Co and Al second coatingcomposition of this invention.

EXAMPLES 3-6 Results similar to those obtained in Example 1 are obtainedby applying a Ta coating and subsequently a Co and Al coatingcomposition, in the precise manner described in Example 1, onto Ni-basealloys having the compositions set forth below. This procedure resultsin the formation of a coating similar to that formed in Example 1, whichconsists essentially of Co, Al, Ta and the particular Ni-base alloyselected:

Ni-19.5Cr-13.5Co-0.7C-3Ti-1.4Al-4Mo-0.005B-0.08Zr

EXAMPLES 7-8 Results similar to those of Example 2 are obtained byapplying first Ta, and then Co and Al coating compositions, by theprocedures set forth in Example 2 to the following Co-base alloys,resulting in the production of a coating zone on these alloys consistingessentially of Co, Al, Ta, and the selected base alloy of the substrate:

We claim:

1. An article of manufacture having good oxidation and erosionresistance at high temperatures which comprises: a substrate selectedfrom the group consisting of Ni-base alloys and Co-base alloys; and anoxidation and erosion resistant surface coating zone comprising Ta, Aland the selected alloy of the substrate adherently bonded to thesubstrate, said surface zone having an exterior surface film of A1 and aTa-rich subzone adjacent to the substrate.

2. The article of claim 1 in which the surface zone contains from 0.1 to1.0% by weight of Ta.

3. An article of manufacture having good stress-rupture strength at hightemperatures, high-temperature oxidation resistance and resistance tocyclic thermal fatigue failure, which comprises: a substrate consistingessentially of an alloy selected from the group consisting of Ni-basealloys and Co-base alloys, and an oxidation and erosion resistantsurface coating zone adherently bonded to the substrate, the surfacezone having an exterior surface film of A1 0 and a Ta-rich subzoneadjacent to the substrate, said surface zone consisting essentially ofCo, Al, Ta and the selected alloy of the substrate.

4. The article of claim 3 in which the selected alloy of the substrateis a Ni-base alloy having Ni as its principal component.

5. The article of claim 3 in which the selected alloy of the substrateis a Co-base alloy having Co as its principal component.

6. The article of claim 3 in which the surface coating zone consistsessentially of: Co and the selected alloy of the substrate, inaggregate, in an atomic ratio to Al between 215 and 1:1; and 0.l%1.0% byweight of the surface zone of Ta.

7. The article of claim 6 in which the coating zone has a thickness of 2to 6.5 mils.

8. The article of claim 6 in which the selected alloy of the substrateis a Ni-base alloy having Ni as its principal component.

9. The article of claim 6 in which the selected alloy of the substrateis a Co-base alloy having Co as its principal component.

References Cited UNITED STATES PATENTS 2,987,423 6/ 1961 Sternberg.

3,000,755 9/1961 Hanink et al. 1l7-13l X 3,054,694 9/ 1962 Aves.

3,096,160 7/1963 Puyear 29197 3,102,044 8/1963 Joseph l17131 X 3,141,7447/ 1964 Couch et al. 29l94 3,330,633 11/1967 Joseph et al. 29l94 ALFREDL. LEAVITT, Primary Examiner J. R. BATIEN, JR., Assistant Examiner US.Cl. X.R.

